50% Fuel to energy conversion 2 stroke
 

The Wartsila-Sulzer RTA96-C turbocharged two-stroke diesel engine is the most powerful and most efficient prime-mover in the world today. The Aioi Works of Japan's Diesel United, Ltd built the first engines and is where some of these pictures were taken.

It is available in 6 through 14 cylinder versions, all are inline engines. These engines were designed primarily for very large container ships. Ship owners like a single engine/single propeller design and the new generation of larger container ships needed a bigger engine to propel them.

The cylinder bore is just under 38" and the stroke is just over 98". Each cylinder displaces 111,143 cubic inches (1820 liters) and produces 7780 horsepower. Total displacement comes out to 1,556,002 cubic inches (25,480 liters) for the fourteen cylinder version.
Some facts on the 14 cylinder version:
Total engine weight: 2300 tons (The crankshaft alone weighs 300 tons.)
Length: 89 feet
Height: 44 feet
Maximum power: 108,920 hp at 102 rpm
Maximum torque: 5,608,312 lb/ft at 102rpm

Fuel consumption at maximum power is 0.278 lbs per hp per hour (Brake Specific Fuel Consumption). Fuel consumption at maximum economy is 0.260 lbs/hp/hour. At maximum economy the engine exceeds 50% thermal efficiency. That is, more than 50% of the energy in the fuel in converted to motion.
For comparison, most automotive and small aircraft engines have BSFC figures in the 0.40-0.60 lbs/hp/hr range and 25-30% thermal efficiency range.

Even at its most efficient power setting, the big 14 consumes 1,660 gallons of heavy fuel oil per hour.

https://people.bath.ac.uk/ccsshb/12cyl/

We just need one in a 1L version.

The main reason for the efficiency of this engine is an extrememly low surface area to displacement ratio. The low ratio is caused by the huge cylinders. Check out this spreadsheet:

1 liter 1 cyl 1 liter 4 cylinder Sulzer
bore 10.8383 6.83 96.52 cm
stroke 10.8383 6.83 248.92 cm
volume per cylinder 1000 250 1821310 cm3
area per cylinder 185 73 14634 cm2
number of cylinders 1 4 1
surface area per liter 0.18 0.29 0.01 cm2

A 1 liter four cylinder engine has 29 times higher surface area per liter than the Sulzer engine! Too much heat leaks out of the cylinders during the power stroke andf reduces efficiency in small engines.

A 1 liter engine can't attain such a good ratio.

However, if manufacturers want to build a 1 liter engine with the lowest possible surface area, it would be a 1 cylinder engine with 108 mm bore and stroke. for best efficicny, there are other considerations, such as achievable compression ratio, that would favor building a slightly smaller bore and longer stroke, say 105 by 115 mm.

But, all other things being equal, a one cylinder car will get the best mileage for a given engine displacement.

They shake too much so it has to be a three cylinder to attain the most reasonable balance . . . why the car makers keep building 4 cylinders I DO NOT KNOW!

Yup, a one cylinder is certainly not a smooth engine. But an opposed twin is.... Just ask a BMW. An opposed twin would be reasonably smooth, and would get better mileage than a similar four or three.

I'm partial to 3 bangers. I have Triumph triple 955i. Love the torque, love the sound. I just thought the article was cool. That's why I posted it.

You can get an old Lister diesel with a big stroke. One of these can run a generator and heat your house on waste heat running on used grease.

I want one of those 2300 ton engines for my Suzuki Swift! Think I would have to beef up the suspension?

The 3 cylinder Metro engine is not as smooth as the 4 cylinder. I had a 3 cylinder Subaru Justy that wasn't bad. It had a balancing shaft to smooth it out. I think the smoothest running (used to be common) engine is the straight 6.

Q

Someone double check my conversion, but I think my TDI's factory published max BSFC efficiency of 197g/KwH works out to 0.326 lb/hp hour, and significantly lower than the quote for most automotive engines, actually closer to the .26 than the .4.
I have absolutely no pretence that my mpg per ton comes anywhere near that of a container ship.

I decided to not get a BMW boxer specifically because of the side to side shake.
Both pistons stopped at TDC or BDC at the same time make the internal reciprocating mass of both pistons stop reciprocating for a moment, just the same as a single cylinder piston. But, instead of the cylinder pressure pushing the head, and attached cylinder and block up as the single piston is pushed down on a power stroke each 720 degrees, the opposed twin first pushes to the engine to the left as that left piston is fired to the right, and then 360 degrees later the engine is pushed right as the right piston is fired left. One pulse in one direction each two rotations, or two pulses, in opposite directions, each two rotations. I don't like getting slapped around, much less on both cheeks.

Instead I bought an 1100 Gold Wing, but even its flat four had a slight rotational shake as the rotating/reciprocating mass ratio changed throughout the crank's rotation (just like my Subarus). I later bought a 1500 Gold Wing with its six which eliminated that issue.

A six is smooth because it is two inline triples, there is a constant momentum to the reciprocating mass, as one piston is stationary at TDC or BDC, the alternate two are near their maximum stroke velocity. When one is at maximum, the other two are nearer to their slowest. A six is smoother than a triple because there are 6 power pulses spaced at 120 degrees rather than 3 at 240. But a triple can be inherently smooth as does not have the same inertial imbalance as a single, twin or four simply because all the reciprocating mass isn't stopped at some given instant.

BUT........ this is a fuel economy forum, not a vibration forum. My point is that a single cylinder will get better mileage than a three or four or six of equivalent displacement. And that an opposed twin reasonable compromise for a multicylinder.

Direct injection, Throttle body, Multi point fuel injection, single or multiple carburetor?
A single cylinder engine will have twice the pulsation of the air flow in the intake as the comparable displacement twin. The start-stop air flow will allow more opportunity for the fuel droplets to settle and coalesce on the wall as the air motion ceases for 630 degrees of the 720 four stroke crank rotation.
A multi cylinder engine will maintain a more constant air flow even if the direction of that air in a manifold changes from cylinder to cylinder.
This is less of a theoretical consideration with direct injection or multi-point injection with an injector dedicated to each cylinder, but in TBI or carburetor engines it needs to be aknowledged.
Internal friction will be less with a single than a twin or higher of same displacement since valves will still have some friction force against a camshaft, piston rings swept area won't diminish by the same factor their number increases, and on, so you do have a point there.
However, an opposed twin is less efficient than an inline twin, an opposed four is less so than an inline four when mining to recycling economy is considered. Materials and machining to make separate heads and cylinders are higher with opposed design engines. Added space requirements from not sharing cylinder walls also make for a more energy and material intensive manufacturing by requiring require a larger vehicle to enclose the engine.
V design multi cylinder engines can offset part of the space requirements, and offer other benefits, that may mitigate the manufacturing increases of two heads. Narrow angle V engines, if narrow enough, can use a single head for both cylinder banks.